Velocity microphone



Jan. 11, 1955 H. F. oLsoN E-r AL 2,699,474

` VELOCITY MICROPHONE Filed Dec. 29, 1950 FUI 7 Z AT'TORNEY United States Patent() VELOCITY MICROPHONE Harry F. Olson, Princeton, and John Preston, Metedeconk, J., assignors to Radio Corporation of America, a corporation of Delaware Application December 29, 1950, Serial No. 203,301 5 Claims. (Cl. 179-1155) electroacoustical apparticularly to a microphone of the force which may be conveyed to suitable amplifying ap-` paratus. In prior art microphones, the magnetic field is usually set up by a permanent magnet or an electromagnet provided with pole pieces arranged in spaced apart relation to provide an air gap within which a conductor, such as a ribbon conductor, may be disposed for movement in response to sound waves impinging thereon.

The magnetic structure plays an important part in the operation of the microphone in at least two different ways. It influences sensitivity since it determines the flux density in the air gap. As a result, it has been determined that the efficiency of magnetic structures increases as the magnetic source is placed closer to the air gap since the amount `of leakage flux decreases. It also otherwise effects the acoustic performance of the microphone since the pole pieces diffract sound waves impinging thereon..

The vibrating element in a velocity microphone is actuated by the difference in sound wave pressure between the front and back surfaces of the element. The pressure, p1, on the front side (that is, the side facing the impinging 'ond wave) of the vibrating element may be written as o ows:

p1=pm sin 100M-An: cos 0) (1) where pm==maximum sound pressure, in dynes per square centimeter,

}\=wavelength, in centimeters,

c=velocity of sound, in centimeters per second :time

Ax=acoustic distance between the front and the back of the vibrating element in centimeters, and

6=angle between the direction of the incident sound and the line normal to the element.

The pressure on the back side of the vibrating element following the pattern of Equation 1 is as follows:

The difference in pressure between the two sides of the vibrating element is given by the equation:

The driving force, in dynes, available for driving the element is given by the equation:

j=SAp=2Sp, cos (kct) sin cos 9) (4) 2,699,474 Patented Jan. 11, 1955 where S.=area of the element, in square centimeters.

Equation 4 shows that when the quantity is small the driving force is proportional to the frequency. Therefore, if a mass controlled element is employed the velocity will be independent of the frequency. When the quantity rs the driving force is zero. This is the cutoff frequency. Therefore, the cutoff frequency is inversely proportional to Ax, the acoustic distance between the front and back of the vibrating element. This distance is determined by the size of the pole pieces. Therefore, in a Velocity microphone with moderate frequency range, the pole pieces can be made quite large.

It is, therefore, a primary object of the present invention to provide an improved microphone of the type described having increased magnetic efficiency.

A further object of the present invention is to provide an improved microphone of the type described which provides good response over a wide frequency range.

Another object of the present invention is to provide a new, improved and simplified structure for a microphone of the type described.

A still further object of the present invention is to provide an improved magnetic field structure for microphones of the type described which will increase the magnetic efficiency thereof.

It is also an object of the present invention to provide a new and simplified support for the conductor element of a velocity type microphone.

Another object of the present invention is to provide an improved structure for a velocity type microphone which is simple in construction, inexpensive in cost, and highly ecient in use.

The velocity microphone of the present invention comprises a magnetic field structure having two magnets disposed in spaced apart relation to provide a magnetic field within which a ribbon conductor is mounted for movement in response to sound waves impinging thereon. This magnetic field, in accordance with the present invention, is provided between opposed ends of the magnets rather than between separate pole pieces excited by the magnets. The supporting framework for the microphone elements is made part of the magnetic circuit and serves as a return path for magnetic ux. In addition thereto, a portion of the framework also serves as part of the Wall structure of a casing used to provide a magnetic shield around the microphone coupling transformer.

The novel features characteristic of the present invention, as well as additional objects and features thereof, will be better understood from the following description of a single, preferred embodiment of a velocity microphone when read in connection with the accompanying drawing in which,

Figure 1 is a front elevation of a velocity microphone, in accordance with the present invention, a portion of the protective screen being broken away to show the internal microphone structure,

Figure 2 is a side elevation, in section, the line 2 2 of Figure 1, and

Figure 3 is an enlarged top plan view of one of the ribbon supports.

Referring more particularly to the drawing wherein similar reference characters designate corresponding parts throughout, there is shown a velocity type microphone 1 having a support member 3 attached to the base thereof and by means of which the microphone may be mounted on a stand or other suitable support 5. The microphone 1 comprises generally, a magnetic field structure 6, a

taken along ribbon conductor or diaphragm 7, a coupling transformer fof permanent magnetic material, are attached to the 3'y inside surface 17 of the frame by any suitable means such as solder or a weld'19`f The magnets 115U are disposed opposite each other with their adjacent faces or edges 21 in spaced apart relation to provide an airgap 23 therebetween; The frame iswmade of'magneticall'y' permeable material so'that it will serve as part of the magnetic circuit and provide 'ai return path for magnetic flux.

The ribbon conductor 7 comprises a thin, corrugated member of .aluminum or otherl suitable material. rIhe' ribbon` 7v is suspended within the airv gap 21 for vibra tory' movement in response to sound waves impinging on the surfaces thereof.

A resilient clip 25 of nonfmagnetic but conductive material, such as Phosphor bronze, is attached to the magnets at each end of the air gap 21 and serves to support the ribbon `conductor 7 within the air-gap: Each clip yZShas an .attaching base 27', an upright portionr 2.9" extendingy atfrght angles fromtheiy base, and a gripping.

orfspring tensioning portion 31 extending from thezupi right portion. Thegripping. portion 31 is` turnedtbaclc upon the upright portion to form a resilient gripping` arm. The gripping portion 31 is` somewhat curved so' that only aportion adjacenty its free -end A33 is` biased into contact with the upright portionV are mounted on the magnets 15 preferably by solden ingor welding, or by any other suitable'mountingrneans. 'Ihe clips also serve as terminals for the yleads 35 con necting the ribbon to the primary coil of the ycoupling: transformer 11. Inasinuch as the clips andy magnets are electrically conductive, it is necessary that' `at least; one of the clips be insulated from the magnets 15-*to4 which it is. attached.

The ribbon conductor 7 is mounted Abytiiisertingl each end( thereof between-the gripping portion 31 andthe tip-- right portion 29 of the clin. Since the gripping portiony isvresilient, centering of the ribbon within thefair gap is facilitatedi In--ordcr to insure good electrical contact between the ribbon and the clamp,- it isdesirable: to-fold the end of. the ribbon over the edge of the-clip uprightportion 29 and solder it in place, in :the-manner shownl more particularly in the drawing;

In order to provide a short sound wave'path length between opposite sidesofthe ribbon;r thereby'tofextend response of the microphone in thev high frequency-range: openings 36 are provided'` in the magnets 15.k The Aopenings 36 should ybe lof la size determined accordingfto the frequency range to be compensated for and may be varied according. to the ycondition to be corrected; For

example, Equation 4 above shows that the-driving: force is-a4 function of the distance .between the-.fronty and back. surfaces `of .the ribbon elements. ThisA distance is vde-- termined by the dimensions ofthe pole pieces.. Iftherel ishahole in the pole pieces: the sound. wave path dis? tance-from the front. to the back of theY ribbon: will' be: decreased. If the hole islarge, so thaty there isvery little acoustic impedance to the transmission of sound, the acoustic pathV from the front to the-back surfaces will-bev through theholes. .Tf the Vholes are offrnoderate size the acoustic distance will be a combination of the paths-aroundV the pole pieces and through theholes.

The coupling'transformer 1l is mounted on the lower end 37 of the frame 13 within the space: between `the magnets 1.5 andrthe sides of the frame; The transformer 1'1` isshielded by mounting it within a steel -case 39s Wax, or other suitable moisture 'proofing material, is filled in around the transformer. The construction of the case is simplified bv torrriinrf the ysides t3-from a section of square steel pipe and utilizing` a section` of the frame lower end 37- toy form theI bottom. A- steel cover is provided which also serves as a. brarket'f'or` holding the perforated protective screens: 12` for kthe microphone in place. Two bolts or vscrews 47 are emlployed to hold'the'cover in placek as well` as tol hol'dfthetransformer and its case in place on the frame 1='3. A terminal board' 49 is mounted on the cover '43 forV convenience in making electrical connections. The two boltsf 47 which hold the cover `also hold the terminal board-in place'.

From the foregoing description, it will be evident that a simplified structure is provided for a velocity type microphone'. The'supporting frame is employed to serve not only as part of the case for shielding the transformer but' also asl part of the magnetic circuit. By'

The clipsV apesar@ having the magnets serve as pole pieces, the percentage ousefulux in" the air' gap is' increased' thereby enhancing the efficiency of the magnetic structure. As a result thereof, one microphone constructed in accordance with the present invention employed only one fourth of a pound of permanent magnet material which produced an average flux density along the length of the air gap of 6100 gausses. The improved typeof ribbon support not only facilitates centering ofthe ribbon conductor but .it isv cheap to manufacture since it can. be stamped4` fromL a single .piece oftsheeL material. If `made from' Phosphor bronze, the lightweightmakesthe. soldering process very simple and easy, as contrasted to heavy machinedfribboni supports' employed in. prior art. microphones, because the amount of heat energy required is less. In addition^,.thelribbont .supports require no tapping or machining to attach them to the magnets, which, in some types of permanent magnet material, is extremely diicult..

Although there: is rshownl but `a single', preferred em-' bodiment of the present invention, it will be.- apparent' tofthosexskillcdA in: the art; thatvarious changes und modifications are. possible within' the spirit of the invention; For'exla.inp1e,. although the supporting frame is shown'v as rectangular, other shapes .may be found more suitable. It' may also.- beffound preferablerfto employ a plurality of. apertures. in each vmagnet insteady ofl the single 'aper-V ture. 4shown Fin order to-shorten 'the' soundwave path length .front to.- baclc of the ribbon. likey nature are possible within` the 'spirit ofV the present invention. Therefore, it ist desired that' the particular formy of= .the -presentinventionl described herein shall be consideredrillustrative and not as; .limiting What isuclaiinedfis:

l... Soundy translating apparatus 'comprising a ypair of' magnets' disposed in .spaced apart :relation to provide a' magnetic'. field .between` opposed ends'of 'said imagnets, a' conductive -ielement disposedy in said" field'l vbetween said" endsis-ofv saidtmagnetst and mounted" for movement' in respensey :to-- sound vwaves impinging' thereon, a` support'- ingzframelconnecting saidV magnets and 'providing' 'a flux' return...path-for said magnets, and Vresilient means attachedto said` magnets and disposed in position to sup- `port. saidiz-conductive element at opposite ends thereof within-.said .magnetic eld, said' resilient vmeansA beingl constituted of electrically conductive material andproviding a terminal for connecting said conductive element tol external amplifyingl 'apparatus..

2i Sound translatingrapparatus comprising a pair of magnets disposed in spaced apart relation to provide a,

magnetic eld between opposed ends of said magnets, a conductive element disposed in said field between said ends .of said `inagnetsi and .mounted for movement inresponsc to sound waves impinging thereon, each of said magnets being provided with at least one opening extending therethrough for :providing `a short sound wave path` length between opposite. sidesi of saidcoriductive element, and a supporting frame directlywcoirnectingsaid magnetsV andi providinga flux return path for said magnets.

3.. Sound `translatingl apparatuscomprising a pair of' magnets disposed in spaced. apart relation to provide a magnetic eld between opposed ends of said magnets, a conductive element disposed. in. said field between said ends of; said .magnets and mounted. for movementv in response `to soundwaves impinging thereon, each of said magnets being provided with at least one opening extending therethmughuforprovidin'g atshort sound wave path length between opposite sides of said conductive clement, a supporting frame directly connected to said magnets andproviding aux return path for :said magnets, `and a coupling. transformer electrically connected 'to said conductive element and a casing for said transformer, said casing including a portion of said supporting frame as 'ipart of itsI wall, structurev and` .providing al magnetic shield around said transformer.

4. In sound translating apparatus having a magnetic type pick-.up unit vincluding spacediapartA pole pieces-providingan air gap and a .conductive ribbonadisposed insaid air gap for movement in response to sound waves impinging thereon, a resilient support for said conductive ribbon comprising n.clipv of non-.magneticfmaterial* attached' to said pole pieces at opposite ends of said air gap, said clips having spring tensioning portions for Other 'changes of receiving and holding the ends of said conductive ribbon thereby to support said ribbon in said air gap.

f' 5. The invention as defined in claim 4 and wherein said clips are made from a single fiat strip of resilient material, each of said clips comprising an attaching base, an upright portion extending from said base and a gripping portion extending from said upright portion, said gripping portion having a free end normally biased into contact with said upright portion.

References Cited in the file of this patent UNITED STATES PATENTS Carlson June 17, 1930 Swickard Sept. 21, 1937 Anderson Dec. 12, 1939 Harry et al. Jan. 7, 1941 Williams et a1. May 20, 1941 Duncan Aug. 29, 1950 

